U.S. patent number 5,933,506 [Application Number 08/441,988] was granted by the patent office on 1999-08-03 for transmitter-receiver having ear-piece type acoustic transducing part.
This patent grant is currently assigned to Nippon Telegraph and Telephone Corporation. Invention is credited to Shigeaki Aoki, Hiroyuki Matsui, Kohichi Matsumoto, Kazumasa Mitsuhashi, Yutaka Nishino, Chikara Yuse.
United States Patent |
5,933,506 |
Aoki , et al. |
August 3, 1999 |
Transmitter-receiver having ear-piece type acoustic transducing
part
Abstract
Ear-piece type acoustic transducing part is provided with a
bone-conducted sound pickup microphone for picking up a
bone-conducted sound, a directional microphone for picking up an
air-conducted sound and an electro-acoustic transducer for
transducing a received speech signal to a received speech sound. A
transmitting-receiving circuit connected to the acoustic
transducing part includes: a low-pass filter which permits the
passage therethrough of low-frequency components in a
bone-conducted sound signal from the bone-conducted sound pickup
microphone; a high-pass filter which permits the passage
therethrough of high-frequency components in an air-conducted sound
signal from the directional microphone; first and second variable
loss circuits which impart losses to the outputs from the low-pass
filter and the high-pass filter, respectively; a comparison/control
circuit which compares the output levels of the low-pass filter and
the high-pass filter with predetermined first and second reference
levels, respectively, and based on the results of comparison,
controls losses that are set in the first and second variable loss
circuits; and a combining circuit which combines the outputs from
the first and second variable loss circuits into a speech sending
signal.
Inventors: |
Aoki; Shigeaki (Yokosuka,
JP), Mitsuhashi; Kazumasa (Zushi, JP),
Nishino; Yutaka (Miura, JP), Matsumoto; Kohichi
(Yokosuka, JP), Yuse; Chikara (Yokosuka,
JP), Matsui; Hiroyuki (Yokohama, JP) |
Assignee: |
Nippon Telegraph and Telephone
Corporation (Tokyo, JP)
|
Family
ID: |
26444359 |
Appl.
No.: |
08/441,988 |
Filed: |
May 16, 1995 |
Foreign Application Priority Data
|
|
|
|
|
May 18, 1994 [JP] |
|
|
6-103766 |
Aug 29, 1994 [JP] |
|
|
6-203977 |
|
Current U.S.
Class: |
381/151; 381/122;
381/94.1; 381/57 |
Current CPC
Class: |
H04R
1/46 (20130101); H04R 3/005 (20130101); H04R
2460/13 (20130101) |
Current International
Class: |
H04R
1/00 (20060101); H04R 3/00 (20060101); H04R
1/46 (20060101); H04R 025/00 () |
Field of
Search: |
;381/92,94,98,111,122,101,102,104,119,121,151,57,94.1,68.3,68.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0 481 529 |
|
Apr 1992 |
|
EP |
|
0 594 063 |
|
Apr 1994 |
|
EP |
|
0108419 |
|
Sep 1978 |
|
JP |
|
WO 94/24834 |
|
Oct 1994 |
|
WO |
|
Primary Examiner: Kuntz; Curtis A.
Assistant Examiner: Nguyen; Duc
Attorney, Agent or Firm: Pollock, Vande Sande &
Amernick
Claims
What is claimed is:
1. A transmitter-receiver comprising:
acoustic transducing means composed of a bone-conducted sound
pickup microphone for picking up a bone-conducted sound and for
outputting a bone-conducted sound signal, a directional microphone
for picking up an air-conducted sound and for outputting an
air-conducted sound signal, and a receiver for transducing a
received speech signal to a received speech sound;
a low-pass filter which permits the passage therethrough of those
low-frequency components in said bone-conducted sound from said
bone-conducted sound pickup microphone which are lower than a
predetermined cutoff frequency;
a high-pass filter which permits the passage therethrough of those
high-frequency components in said air-conducted sound from said
directional microphone which are higher than said cutoff
frequency;
first and second variable loss circuits which impart losses to the
outputs from said low-pass filter and said high-pass filter;
a comparison/control circuit which compares the output levels of
said low-pass filter and said high-pass filter with predetermined
first and second reference levels and, based on the results of
comparison, controls the losses that are set in said first and
second variable loss circuits;
a combining circuit which combines the outputs from said first and
second variable loss circuits and outputs a speech sending signal;
and
means for supplying said received speech signal to said
receiver.
2. The transmitter-receiver of claim 1, wherein said acoustic
transducing means includes an omnidirectional microphone for
detecting noise components, and which further comprises a noise
suppressing part which combines the outputs from said directional
microphone and said omnidirectional microphone to suppress said
noise components and supplies said high-pass filter with said noise
component suppressed output.
3. A transmitter-receiver comprising:
acoustic transducing means composed of a bone-conducted sound
pickup microphone for picking up a bone-conducted sound, a
directional microphone for picking up an air-conducted sound, an
omnidirectional microphone for detecting noise, and a receiver for
transducing a received speech signal to a received speech
sound;
a low-pass filter which permits the passage therethrough of those
low-frequency components in the output from said bone-conducted
sound pickup microphone which are lower than a predetermined cutoff
frequency;
a noise suppressing part which combines the outputs from said
directional microphone and said omnidirectional microphone to
suppress a noise component;
a high-pass filter which permits the passage therethrough of those
high-frequency components in the output from said noise suppressing
part which are higher than said cutoff frequency;
a combining circuit which mixes the outputs from said low-pass
filter and said high-pass filter and outputs a speech sending
signal; and
means for supplying said received speech signal to said
receiver.
4. The transmitter-receiver of claim 1, which further comprises:
third and fourth variable loss circuits connected to the output
side of said combining circuit and the input side of said received
speech signal supplying means, for controlling the levels of said
speech sending signal and said received speech signal,
respectively; and a second comparison/control circuit which
compares the level of said speech sending signal to be fed to said
third variable loss circuit and the level of said received speech
signal to be fed to said fourth variable loss circuit with
predetermined third and fourth reference levels, respectively, and
on the basis of the results of comparison, controls the losses that
are set in said third and fourth variable loss circuits.
5. The transmitter-receiver of claim 2 or 3, wherein said noise
suppressing part comprises: a first amplifier for amplifying said
air-conducted sound signal from said directional microphone; a
second amplifier for amplifying said noise components from said
omnidirectional microphone; and a noise suppressor circuit which
adds together the outputs from said first and second amplifiers in
a 180.degree. out-of-phase relation to each other to generate an
air-conducted sound signal with said noise components suppressed
and applies it to said high-pass filter.
6. A transmitter-receiver comprising:
acoustic transmitting means composed of a bone-conducted sound
pickup microphone for picking up a bone-conducted sound and for
outputting a bone-conducted sound signal, air-conducted sound
pickup microphone means for picking up an air-conducted sound and
for outputting an air-conducted sound signal, and a receiver for
transducing a received speech signal to a received speech;
comparison/control means which estimates a level of ambient noise,
compares said estimated level with a predetermined threshold level
and generates a control signal on the basis of the results of
comparison; and
speech sending signal generating means which responds to said
control signal to control additive mixing of said air-conducted
sound signal from said air-conducted sound pickup microphone means
and said bone-conducted sound signal from said bone-conducted sound
pickup microphone to generate a speech sending signal;
wherein said comparison/control means generates, as said control
signal, a signal indicating whether said estimated noise level is
higher or lower than said threshold level; said speech sending
signal generating means includes signal select means responsive to
said control means to select either one of said bone-conducted
sound signal and said air-conducted sound signal; and said speech
sending signal generating means generates said speech sending
signal from said selected signal.
7. A transmitter-receiver comprising:
acoustic transmitting means composed of a bone-conducted sound
pickup microphone for picking up a bone-conducted sound and for
outputting a bone-conducted sound signal, air-conducted sound
pickup microphone means for picking up an air-conducted sound and
for outputting an air-conducted sound signal, and a receiver for
transducing a received speech signal to a received speech;
comparison/control means which estimates a level of ambient noise,
compares said estimated level with a predetermined threshold level
and generates a control signal on the basis of the results of
comparison; and
speech sending signal generating means which responds to said
control signal to control additive mixing of said air-conducted
sound signal from said air-conducted sound pickup microphone means
and said bone-conducted sound signal from said bone-conducted sound
pickup microphone to generate a speech sending signal;
wherein said comparison/control means is a means which, when said
estimated noise level is within an area of a fixed width defined
about said threshold level, supplies said speech sending signal
generating means with a control signal for mixing said
air-conducted sound signal and said bone-conducted sound signal at
a ratio corresponding to said estimated noise level; and said
speech sending signal generating means includes a means responsive
to said control signal to mix said air-conducted sound signal and
said bone-conducted sound signal at said ratio.
8. The transmitter-receiver of claim 6, or 7 wherein said
comparison/control means includes means for holding a relationship
between the ambient noise level and at least the level of said
air-conducted sound signal in non-talking states; and said
comparison/control means is a means which obtains, as said
estimated noise level, a noise level corresponding to the level of
the air-conducted sound signal during the use of said
transmitter-receiver based on said relationship, compares said
estimated noise level with said threshold level and generates said
control signal on the basis of the result of comparison.
9. The transmitter-receiver of claim 8, wherein said relationship
is the relationship between the ambient noise level and the level
ratio of said bone-conducted sound signal versus said air-conducted
sound signal; and said comparison/control means includes means
which obtains a level ratio between said bone-conducted sound
signal and said air-conducted sound signal and obtains the noise
level corresponding to said level ratio, as said estimated noise
level, from said relationship.
10. The transmitter-receiver of claim 6 or 7, wherein said
comparison/control means includes means for holding a relationship
between the ambient noise level and at least the level of the
air-conducted sound signal in a talking state; and said
comparison/control means is a means which obtains, as said
estimated noise level, a noise level corresponding to the level of
said air-conducted sound signal during the use of said
transmitter-receiver based on said relationship, compares said
estimated noise level with said threshold value and generates said
control signal on the basis of the result of comparison.
11. The transmitter-receiver of claim 10, wherein said relationship
is the relationship between the ambient noise level and the ratio
of bone-conducted sound signal level versus air-conducted sound
signal level; and said comparison/control means includes means
which obtains a level ratio between said bone-conducted sound
signal and said air-conducted sound signal and obtains the noise
level corresponding to said level ratio, as said estimated noise
level, from said relationship.
12. The transmitter-receiver of claim 6 or 7, wherein said
comparison/control means includes means for holding a first
relationship between the ambient noise level and at least the level
of said air-conducted sound signal in non-talking states and a
second relationship between the ambient noise level and at least
the level of said air-conducted sound signal in a talking state;
and said comparison/control means is a means which compares the
level of said received speech signal and at least one of the level
of said air-conducted sound signal and the level of said
bone-conducted sound signal with predetermined first and second
reference level values, respectively, to determine if said
transmitter-receiver is in a talking or listening state, and on the
basis of said first or second relationship corresponding to the
results of comparison, obtains, as said estimated noise level, a
noise level corresponding to at least said air-conducted sound
signal, compares said estimated noise level with said threshold
value, and generates said control signal on the basis of the result
of comparison.
13. The transmitter-receiver of claim 12, wherein said first and
second relationships are relationships between the ambient noise
level and the level ratio of said bone-conducted sound signal
versus said air-conducted sound signal in a non-talking state and
in a talking state, respectively; and said comparison/control means
includes means which obtains the level ratio between said
bone-conducted sound signal and said air-conducted sound signal and
obtains the estimated noise level corresponding to said level ratio
from either one of said first and second relationships.
14. The transmitter-receiver of claim 6 or 7 which further
comprises first and second signal dividing means for dividing each
of at least said air-conducted sound signal and said bone-conducted
sound sending signal into a plurality of frequency bands; said
speech sending signal generating means comprises a plurality of
signal mixing circuits each of which is supplied with said
air-conducted sound signal and said bone-conducted sound signal of
the corresponding frequency band from said first and second signal
dividing means, then mixes them in accordance with a band control
signal and outputs the mixed signal, and a signal combining circuit
which combines the outputs from said plurality of signal mixing
circuits and outputs the combined signal as said speech sending
signal; and said comparison/control means is a means which is
supplied with at least said air-conducted sound signals of the
corresponding frequency bands from said first signal dividing
means, estimates ambient noise levels of said frequency bands from
said air-conducted sound signals, compares said estimated noise
levels with a plurality of threshold values predetermined for said
plurality of frequency bands, respectively, and generates band
control signals on the basis of the results of comparison.
15. The transmitter-receiver of claim 14, wherein said
comparison/control means includes means for holding a relationship
between said ambient noise levels in said plurality of frequency
bands in non-talking states and at least the levels of said
air-conducted sound signals of the corresponding frequency bands;
and said comparison/control means is a means which obtains, as said
estimated noise level of each frequency band, a noise level
corresponding to the level of the air-conducted sound signal during
the use of said transmitter-receiver based on said relationship,
compares said estimated noise level with said threshold value, and
generates said band control signal of said each frequency band on
the basis of the result of comparison.
16. The transmitter-receiver of claim 15, wherein said relationship
is the relationship between the ambient noise level and the level
ratio of said bone-conducted sound signal versus said air-conducted
sound signal in each frequency band in non-talking states; and said
comparison/control means includes means which obtains a level ratio
between said bone-conducted sound signal and said air-conducted
sound signal in each frequency band and obtains the noise level
corresponding to said level ratio, as said estimated noise level of
said each frequency band, from said relationship.
17. The transmitter-receiver of claim 14, wherein said
comparison/control means includes means for holding a relationship
between ambient noise levels in said plurality of frequency bands
and at least levels of said air-conducted sound signals of the
corresponding frequency bands in talking states; and said
comparison/control means is a means which obtains, as said
estimated noise level of each frequency band, a noise level
corresponding to the level of the air-conducted sound signal during
the use of said transmitter-receiver based on said relationship,
compares said estimated noise level with said threshold value, and
generates said band control signal of said each frequency band on
the basis of the result of comparison.
18. The transmitter-receiver of claim 17, wherein said relationship
is the relationship between the ambient noise level and the level
ratio of said bone-conducted sound signal versus said air-conducted
sound signal for each frequency band in said talking states; and
said comparison/control means includes means which obtains a level
ratio between said bone-conducted sound signal and said
air-conducted sound signal for each frequency band, and obtains the
noise level corresponding to said level ratio, as said estimated
noise level of said each frequency band, from said
relationship.
19. The transmitter-receiver assembly of claim 14, wherein said
comparison/control means includes means for holding a first
relationship between the ambient noise level and at least the level
of said air-conducted sound signal in each corresponding frequency
band in non-talking states and a second relationship between the
ambient noise level and at least the level of said air-conducted
sound signal in a talking state; and said comparison/control means
is a means which compares the level of said received speech signal
and at least one of the level of said air-conducted sound signal
and the level of said bone-conducted sound signal in each frequency
band with predetermined first and second reference level values,
respectively, for said frequency band to determine if said
transmitter-receiver is in a talking or listening state, and on the
basis of said first or second relationship corresponding to the
result of determination, obtains, as said estimated noise level, a
noise level corresponding to at least the level of said
air-conducted sound signal, compares said estimated noise level
with said threshold value, and generates said control signal of
said each frequency band on the basis of the result of
comparison.
20. The transmitter-receiver of claim 19, wherein said first and
second relationships for each frequency band between the ambient
noise level and the level ratio of said bone-conducted sound signal
versus said air-conducted sound signal in a non-talking state and
in a talking state, respectively; and said comparison/control means
includes means which obtains the level ratio between said
bone-conducted sound signal and said air-conducted sound signal for
each frequency band and obtains the estimated noise level
corresponding to said level ratio from either one of said first and
second relationships.
21. The transmitter-receiver of claim 6 or 7, which further
includes a directional microphone and an omnidirectional microphone
as said air-conducted sound pickup microphone and noise suppressing
means, said noise suppressing means being a means which, during a
silent state and a listening state, outputs a signal from said
omnidirectional microphone as said air-conducted sound signal
representing a noise signal and, during a talking state, combines
signals from said directional microphone and said omnidirectional
microphone and outputs said combined signal as said air-conducted
sound signal with noise suppressed.
Description
TECHNICAL FIELD
The present invention relates to a transmitter-receiver which
comprises an ear-piece type acoustic transducing part having a
microphone and a receiver formed as a unitary structure and a
transmitting-receiving circuit connected to the acoustic
transducing part and which permits hands-free communications. More
particularly, the invention pertains to a transmitter-receiver
which has an air-conducted sound pickup microphone and a
bone-conducted sound pickup.
BACKGROUND OF THE INVENTION
Conventionally, this kind of transmitter-receiver employs, as its
ear-piece or ear-set type acoustic transducing part, (1) means
which picks up vibrations of the skull caused from talking sound by
an acceleration pickup set in the auditory canal (which means will
hereinafter be referred to also as a bone-conducted sound pickup
microphone and the speech sending signal picked up by this means
will hereinafter be referred to as a "bone-conducted sound
signal"), or (2) means which guides a speech or talking sound as
vibrations of air by a sound pickup tube extending to the vicinity
of the mouth and picks up the sound by a microphone set on an ear
(which means will hereinafter be referred to also as an
air-conducted sound pickup microphone and the speech sending signal
picked up by this means will hereinafter be referred to as an
"air-conducted sound signal").
Such a conventional transmitter-receiver of the type which sends
speech through utilization of bone conduction is advantageous in
that it can be used even in a high-noise environment and permits
hands-free communications. However, this transmitter-receiver is
not suited to ordinary communications because of its disadvantages,
i.e. the clarity of articulation of the transmitted speech is so
low that the listener cannot easily identify the talker, the
clarity of articulation of the transmitted speech greatly varies
from person to person or according to the way of setting the
acoustic transducing part on an ear, and an abnormal sound as by
the friction of cords is also picked up. On the other hand, the
transmitter-receiver of the type utilizing air conduction is more
excellent in clarity than the above but has defects that it is
inconvenient to handle when the sound pickup tube is long and the
speech sending signal is readily affected by ambient noise when the
tube is short.
The air-conducted sound pickup microphone picks up sounds that have
propagated through the air, and hence has a feature that the tone
quality of the picked-up speech signals is relatively good but is
easily affected by ambient noise. The bone-conducted sound pickup
microphone picks up a talker's vocal sound transmitted through the
skull into the ear set, and hence has a feature that the tone
quality of the picked-up speech signal is relatively low because of
large attenuation of components above 1 to 2 KHz, but the speech
signal is relatively free from the influence of ambient noise. As a
transmitter-receiver assembly for sending excellent speech
(acoustic) signals through utilization of the merits of such
air-conducted sound pickup microphone and bone-conducted sound
pickup microphone, there is disclosed in Japanese Utility Model
Registration Application Laid-Open No. 206393/89 a device that
mixes the speech signal picked up by the air-conducted sound pickup
microphone and the speech signal picked up by the bone-conducted
sound pickup microphone.
According to this device, the speech signals from the bone
conduction type microphone and the air conduction type microphone
are both applied to a low-pass filter and a high-pass filter which
have a cutoff frequency of 1 to 2 KHz, then fed to variable
attenuators and combined by a mixer into a speech sending signal.
With this configuration, low-frequency noises in the output from
the air conduction type microphone which are lower than the cutoff
frequency are removed, and it is possible to remove or cancel
components higher than the cutoff frequency in the noise which the
bone conduction type microphone is likely to pick up, such as noise
produced by friction between a cord extending from the ear set and
the human body or clothing, or wind noise produced by wind blowing
against the ear set. Moreover, in a high-noise environment, the SN
ratio of the speech sending signal can be improved by decreasing
the attenuation of the bone-conducted sound signal from the
low-pass filter and increasing the attenuation of the air-conducted
sound signal from the high-pass filter through manual control.
With this configuration, however, when the level of noise from the
air-conducted sound pickup microphone is high, frequency components
higher than the cutoff frequency need to be appreciably attenuated
for the purpose of attenuating the noise, and consequently, the
speech sending signal is substantially composed only of the
bone-conducted sound signal components, and hence is extremely low
in tone quality. Moreover, the attenuation control by the variable
attenuator is manually effected by an ear set user and the user
does not monitor the speech sending signal; hence, it is almost
impossible to set the attenuation to the optimum value under
circumstances where the amount of noise varies. Furthermore, it is
cumbersome to manually control the ratio of combining the speech
signal from the air-conducted sound pickup microphone and the
speech signal from the bone-conducted sound pickup microphone.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
transmitter-receiver which automatically processes the speech
sending signal in accordance with use environments (such as the
tone quality and the amount of sound) to send speech of the best
tone quality.
The transmitter-receiver according to a first aspect of the present
invention is constructed so that it comprises: an acoustic
transducing part including a bone-conducted sound pickup microphone
for picking up a bone-conducted sound and for outputting a
bone-conducted sound signal, a directional microphone for picking
up an air-conducted sound and for outputting an air-conducted sound
signal, and a receiver for transducing a received speech signal to
a received speech sound; a low-pass filter which permits the
passage therethrough of those low-frequency components in the
bone-conducted sound from the bone-conducted sound pickup
microphone which are lower than a predetermined cutoff frequency; a
high-pass filter which permits the passage therethrough of those
high-frequency components in the air-conducted sound from the
direction microphone which are higher than the above-mentioned
cutoff frequency; first and second variable loss circuits which
impart losses to the outputs from the low-pass filter and the
high-pass filter, respectively; a comparison/control circuit which
compares the output levels of the low-pass filter and the high-pass
filter with predetermined first and second reference level values,
respectively, and based on the results of comparison, controls the
losses that are set in the first and second variable loss circuits;
a combining circuit which combines the outputs from the first and
second variable loss circuits into a speech sending signal; and
means for supplying the received speech signal to the receiver.
The transmitter-receiver according to the first aspect of the
invention may be constructed so that the acoustic transducing part
includes an omnidirectional microphone for detecting a noise
component, and the transmitter-receiver further comprises a noise
suppressing part which suppresses the noise component by combining
the outputs from the directional microphone and the omnidirectional
microphone and supplies the high-pass filter with the combined
output having canceled therefrom the noise component.
The transmitter-receiver according to a second aspect of the
present invention is constructed so that it comprises: an acoustic
transducing part including a bone-conducted sound pickup microphone
for picking up a bone-conducted sound, a directional microphone for
picking up an air-conducted sound, an ommidirectional microphone
for detecting noise and a receiver for transducing a received
speech signal to a received speech sound; a low-pass filter which
permits the passage therethrough of those low-frequency components
in the output from the bone-conducted sound pickup microphone which
are lower than a predetermined cutoff frequency; a noise
suppressing part which combines the outputs from the directional
microphone and the omnidirectional microphone to suppress the noise
component; a high-pass filter which permits the passage
therethrough of those high-frequency components in the output from
the noise suppressing part which are higher than the
above-mentioned cutoff frequency; a combining circuit which
combines the outputs from the low-pass filter and the high-pass
filter into a speech sending signal; and means for supplying the
received speech signal to the receiver.
The transmitter-receiver assembly according to the first or second
aspect of the invention may be constructed so that it further
comprise: third and fourth variable loss circuits connected to the
output side of the combining circuit and the input side of the
received speech signal supplying means, for controlling the levels
of the speech sending signal and the received speech signal,
respectively; and a second comparison/control circuit which
compares the level of the speech sending signal to be fed to the
third variable loss circuit and the level of the received speech
signal to be fed to the fourth variable loss circuit with
predetermined third and fourth reference level values,
respectively, and based on the results of comparison, controls the
losses that are set in the third and fourth variable loss
circuits.
The transmitter-receiver according to a third aspect of the present
invention is constructed so that it comprises: an acoustic
transducing part including a bone-conducted sound pickup microphone
for picking up a bone-conducted sound and for outputting a
bone-conducted sound signal, an air-conducted sound pickup
microphone for picking up an air-conducted sound and for outputting
an air-conducted sound signal, and a receiver for transducing a
received speech signal to a received speech sound;
comparison/control means which estimates the level of ambient
noise, compares the estimated ambient noise level with a
predetermined threshold value and generates a control signal on the
basis of the result of comparison; and speech sending signal
generating means which responds to the control signal to mix the
air-conducted sound signal from the air-conducted sound pickup
microphone and the bone-conducted sound signal from the
bone-conducted sound pickup microphone in accordance with the
above-mentioned estimated noise level to generate a speech sending
signal.
The transmitter-receiver according to the third aspect of the
invention may be constructed so that the comparison/control means
includes means for holding a relationship between the ambient noise
level and at least the level of the air-conducted sound signal in
non-talking states, and the comparison/control means obtains, as
said estimated noise level, a noise level corresponding to the
level of the air-conducted sound signal during the use of said
transmitter-receiver based on said relationship, compares the
estimated noise level with the above-mentioned threshold value, and
generates the control signal on the basis of the result of
comparison.
The transmitter-receiver according to the third aspect of the
invention may also be constructed so that the comparison/control
means includes means for holding a relationship between the ambient
noise level and at least the level of the air-conducted sound
signal in the talking state, and the comparison/control means
obtains, as said estimated noise level, a noise level corresponding
to the level of the air-conducted sound signal during the use of
said transmitter-receiver based on said relationship, compares the
estimated noise level with the threshold value, and generates the
control signal on the basis of the result of comparison.
The transmitter-receiver according to the third aspect of the
invention may also be constructed so that the comparison/control
means includes means for holding a first relationship between the
ambient noise level and at least the level of the air-conducted
sound signal in the non-talking state and a second relationship
between the ambient noise level and at least the level of the
air-conducted sound signal in the talking state, and the
comparison/control means compares the level of the received speech
signal and at least one of the level of the air-conducted sound
signal and the level of the bone-conducted sound signal during the
use of the transmitter-receiver with predetermined first and second
reference level values, respectively, to determine if the
transmitter-receiver is in the talking or listening state, and
based on the first or second relationship corresponding to the
result of determination, obtains, as said estimated noise level, a
noise level corresponding to at least the level of the
air-conducted sound signal, then compares the estimated noise level
with the threshold value, and generates the control signal on the
basis of the result of comparison.
The transmitter-receiver according to the third aspect of the
invention may also be constructed so that it further comprises
first and second signal dividing means for dividing the
air-conducted sound signal and the bone-conducted sound signal into
pluralities of frequency bands, the speech sending signal
generating means includes a plurality of signal mixing circuits
each of which is supplied with the air-conducted sound signal and
the bone-conducted sound signal of the corresponding frequency band
from the first and second signal dividing means and mixes them in
accordance with a band control signal, and a signal combining
circuit which combines the outputs from the plurality of signal
mixing circuits and outputs the combined signal as the speech
sending signal, and the comparison/control means are supplied with
the air-conducted sound signals of the corresponding frequency
bands from at least the first signal dividing means, estimates the
ambient noise levels of the respective frequency bands from at
least the air-conducted sound signals of the corresponding
frequency bands, then compares the estimated noise levels with a
plurality of threshold values predetermined for the plurality of
frequency bands, respectively, and generates the band control
signals on the basis of the results of comparisons.
The transmitter-receiver according to the third aspect of the
invention may also be constructed so that it further comprises a
directional microphone and an omnidirectional microphone as the
air-conducted sound pickup microphone means and noise suppressing
means, and the noise suppressing means outputs the signal from the
omnidirectional microphone as the air-conducted sound signal
representing a noise signal during the silent and the listening
state and, during the talking state, combines the signals from the
directional microphone and the omnidirectional microphone and
outputs the combined signal as the air-conducted sound signal with
noise suppressed or canceled therefrom.
As described above, according to the first aspect of the present
invention, a bone-conducted sound composed principally of
low-frequency components and an air-conducted sound composed
principally of high-frequency components are mixed together to
generate the speech sending signal and the ratio of mixing the
sounds is made variable in accordance with the severity of ambient
noise or an abnormal sound picked up by the bone-conducted sound
pickup microphone; therefore, it is possible to implement a
transmitter-receiver which makes use of the advantages of the
conventional bone-conduction communication device, i.e. it can be
used in a high-noise environment and permits hands-free
communications and which, at the same time, obviates the defects of
the conventional bone-conduction communication device, such as low
articulation or clarity of speech and discomfort by abnormal
sounds.
According to the second aspect of the present invention, it is
possible to efficiently cancel the noise component in the
air-conducted sound by the noise component from the
omnidirectionnal microphone and to effectively prevent howling
which results from coupling the speech sending signal and the
received speech signal.
According to the third aspect of the present invention, an
estimated value of the ambient noise level is compared with a
threshold value, then a control signal is generated on the basis of
the result of comparison, and the air-conducted sound signal picked
up by the directional microphone and the bone-conducted sound
signal picked up by the bone-conducted sound pickup microphone are
mixed together at a ratio specified by the control signal to
generate the speech sending signal. Hence, this communication
device is able to send a speech signal of excellent tone quality,
precisely reflecting the severity and amount of ambient noise
regardless of whether the device is in the talking or listening
state.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view illustrating the configuration of an
acoustic transducing part for use in a first embodiment of the
present invention;
FIG. 2 is a block diagram illustrating the construction of a
transmitting-receiving circuit connected to the acoustic
transducing part in FIG. 1;
FIG. 3 is a diagram for explaining the characteristics of a
directional microphone and an omnidirectional microphone;
FIG. 4 is a table for explaining control operations of a
comparison/control circuit 24 shown in FIG. 2;
FIG. 5 is a block diagram illustrating a transmitter-receiver
according to a second embodiment of the present invention;
FIG. 6 is a graph showing the relationship between the tone quality
of an air-conducted sound signal and the ambient noise level, and
the relationship between the tone quality of a bone-conducted sound
signal and the ambient noise level;
FIG. 7 is a graph showing the relationship of the ambient noise
level to the level ratio between the bone-conducted sound signal
and the air-conducted sound signal in the listening or silent
state;
FIG. 8 is a graph showing the relationship of the ambient noise
level to the level ratio between the bone-conducted sound signal
and the air-conducted sound signal in the talking or double-talking
state;
FIG. 9 is a table for explaining operating states of the FIG. 5
embodiment;
FIG. 10A is a block diagram showing the construction of a signal
mixing circuit which is used as a substitute for each of signal
select circuits 33.sub.1 to 33.sub.n in the FIG. 5 embodiment;
FIG. 10B is a graph showing the mixing operation of the circuit
shown in FIG. 10A;
FIG. 11 is a block diagram illustrating a modified form of the FIG.
5 embodiment; and
FIG. 12 is a block diagram showing the comparison/control circuit
32 in FIG. 5 or 11 constructed as an analog circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 there is schematically illustrated the configuration of
an ear-piece type acoustic transducing part 10 for use in an
embodiment of the present invention. Reference numeral 11 denotes a
case of the ear-piece type acoustic transducing part 10 wherein
various acoustic transducers described later are housed, 12 is a
lug or protrusion for insertion into the auditory canal 50, and 13
is a sound pickup tube for picking up air-conduction sounds. The
sound pickup tube 13 is designed so that it faces the user's mouth
when the lug 12 is put in the auditory canal 50; that is, it is
adapted to pick up sounds only in a particular direction. The lug
12 and the sound pickup tube 13 are formed as a unitary structure
with the case 11.
Reference numeral 14 denotes an acceleration pickup (hereinafter
referred to as a bone-conduction sound microphone) for picking up
bone-conduction sounds, and 15 is a directional microphone for
picking up air-conduction sounds (i.e. an air-conduction sound
microphone), which has such directional characteristics that its
sensitivity is high in the direction of the user's mouth (i.e. in
the direction of the sound pickup tube 13). The directional
microphone 15 has its directivity defined by the combining of sound
pressure levels of sound picked up from the front of the microphone
15 and sound picked up from behind through a guide hole 11H.
Accordingly, the directivity could also be obtained even if the
sound pickup tube 13 is removed to expose the front of the
directional microphone 15 in the surface of the case 11.
Reference numeral 16 denotes an omnidirectional microphone for
detecting noise, which has a sound pickup aperture or opening in
the direction opposite to the directional microphone 15. Reference
numeral 17 denotes an electro-acoustic transducer (hereinafter
referred to as a receiver) for transducing a received speech signal
into a sound, and 18 designates lead wires for interconnecting the
acoustic transducing part 10 and a transmitting-receiving circuit
20 described later; the transmitting-receiving circuit 20 has its
terminals T.sub.A, T.sub.B, T.sub.C and T.sub.D connected via the
lead wires 18 to the directional microphone 15, the bone-conduction
sound microphone 14, the receiver 17 and the omnidirectional
microphone 16, respectively.
In FIG. 2 there is shown in block form the configuration of the
transmitting-receiving circuit 20 which is connected to the
acoustic transducing part 10 exemplified in FIG. 1. In FIG. 2
terminals T.sub.A, T.sub.B, T.sub.C and T.sub.D are connected to
T.sub.A, T.sub.B, T.sub.C and T.sub.D in FIG. 1, respectively.
Reference numeral 21B denotes an amplifier for amplifying a
bone-conduction sound signal from the bone-conduction sound
microphone 14, and 21A is an amplifier for amplifying an
air-conduction sound signal from the directional, air-conduction
sound microphone 15. The gains of the amplifiers 21B and 21A are
preset so that their output speech signal levels during a no-noise
period are of about the same order at the inputs of a
comparison/control circuit 24 described later. Reference numeral
21U denotes an amplifier which amplifies a noise signal from the
noise detecting omnidirectional microphone 16 and whose gain is
preset so that its noise output during a silent period becomes
substantially the same as the noise output level of the amplifier
21A in a noise suppressor circuit 23 described later. The
amplifiers 21A and 21B and the noise suppressor circuits 23
constitute a noise suppressing part 20N. The noise suppressor
circuit 23 substantially cancels the noise signal by adding
together the outputs from the amplifiers 21A and 21U after shifting
them 180.degree. out of phase to each other.
Reference numeral 22B denotes a low-pass filter (LPF), which may
preferably be one that approximates characteristics inverse to the
frequency characteristics of the bone-conduction sound microphone
used; but it may be a simple low-pass filter of a characteristic
such that it cuts the high-frequency components of the output
signal from the amplifier 21B but passes therethrough the
low-frequency components, and its cutoff frequency is selected
within the range of 1 to 2 KHz. Reference numeral 22A denotes a
high-pass filter (HPF), which may preferably be one that
approximates characteristics inverse to the frequency
characteristics of the directional microphone 15; but it may be a
simple high-pass filter of a characteristic such that it cuts the
low-frequency components of the output signal from the noise
suppressor circuit 23 and passes therethrough the high-frequency
components, and its cutoff frequency is selected within the range
of 1 to 2 KHz.
The directional microphone 15 and the omnidirectional microphone 16
bear such a relationship of sensitivity characteristic that the
former has a high sensitivity within a narrow azimuth angle but the
sensitivity of the latter is substantially the same in all
directions as indicated by ideal sensitivity characteristics 15S
and 16S in FIG. 3, respectively. Then, assuming that the ambient
noise level is the same in any directions and at any positions, and
letting the total amount of noise energy per unit time applied to
the omnidirectional microphone 16 from all directions be
represented by the surface area N.sub.U of a sphere with a radius
r, the noise energy per unit time applied to the directional
microphone 15 is represented by an area N.sub.A defined by the
spreading angle of its directional characteristic on the surface of
the sphere. Hence, their energy ratio N.sub.A /N.sub.U takes a
value sufficiently smaller than one. Now, assume that the amounts
of speech energy S.sub.A and S.sub.U applied to the directional
microphone 15 and the omnidirectional microphone 16 take the same
value S, and let the gains of the amplifiers 21A and 21U be
represented by G.sub.A and G.sub.U, respectively. By setting that a
value G.sub.A N.sub.A is nearly equal to a value G.sub.U N.sub.U,
noise is substantially canceled by the noise suppressor circuit 23
but the speech signal level at the output of the noise suppressor
circuit 23 becomes G.sub.A S-G.sub.U S=G.sub.A S(1-N.sub.A
/N.sub.U), and since the energy ratio N.sub.A /N.sub.U is
sufficiently smaller than one, the speech level is nearly equal to
G.sub.A S--this indicates that a speech signal in the
air-conduction sound signal can be effectively extracted therefrom
ideally. The noise suppressing effect that could be achieved by the
directional microphone 15, the omnidirectional microphone 16 and
the noise suppressing part 20N actually used was typically in the
range of 3 to 10 dB.
In FIG. 2 the bone-conduction sound signal and the air-conduction
sound signal, which have their frequency characteristics equalized
by the low-pass filter 22B and the high-pass filter 22A,
respectively, are applied to the comparison/control circuit 24,
wherein their levels V.sub.B and V.sub.A are compared with
predetermined reference levels V.sub.RB and V.sub.RA, respectively.
Based on the results of comparison, the comparison/control circuit
24 controls losses L.sub.B and L.sub.A of variable loss circuits
25B and 25A, thereby controlling the levels of the bone- and
air-conducted sound signals. A mixer circuit 26 mixes the
bone-conducted sound signal and the air-conducted sound signal
which have passed through the variable loss circuits 25B and 25A.
The thus mixed signal is provided as a speech sending signal
S.sub.T to a speech sending signal output terminal 20T via a
variable loss circuit 29T. A comparison/control circuit 28 compares
the level of a speech receiving signal S.sub.R and the level of the
speech sending signal S.sub.T with predetermined reference levels
V.sub.RR and V.sub.RT, respectively, and, based on the results of
comparison, controls the losses of variable loss circuits 29T and
29R, thereby controlling the levels of the speech sending signal
and the speech receiving signal to suppress an echo or howling. The
speech receiving signal from the variable loss circuit 29R is
amplified by an amplifier 27 to an appropriate level and then
applied to the receiver 17 via the terminal T.sub.C.
FIG. 4 is a table for explaining the control operations of the
comparison/control circuit 24 in FIG. 2. The comparison/control
circuit 24 compares the output level V.sub.B of the low-pass filter
22B and the output level V.sub.A of the high-pass filter 22A with
the predetermined reference levels V.sub.RB and V.sub.RA,
respectively, and determines if the bone- and air-conducted sound
signals are present (white circles) or absent (crosses), depending
upon whether the output levels are higher or lower than the
reference levels. In FIG. 4, state 1 indicates a state in which the
bone-conducted sound signal (the output from the low-pass filter
23B) and the air-conducted sound signal (the output from the
high-pass filter 23A), both frequency-equalized, are present at the
same time, that is, a speech sending or talking state. State 2
indicates a state in which the bone-conducted sound signal is
present but the air-conducted sound signal is absent, that is, a
state in which the bone-conducted sound pickup microphone 14 is
picking up abnormal sounds such as wind noise of the case 11 and
frictional sounds produced by the lead wires 18 and the human body
or clothing. State 3 indicates a state in which the air-conducted
sound signal is present but the bone-conducted sound signal is
absent, that is, a state in which no speech signal is being sent
and that noise component of the ambient sound picked up by the
directional microphone 15 which has not been canceled by the noise
suppressor circuit 23 is being outputted. State 4 indicates a state
in which neither of the bone- and air-conducted sound signals is
present, that is, a state in which no speech signal is being sent
and no noise is present. The control operations described in the
right-hand columns of the FIG. 4 table show the operations which
the comparison/control circuit 24 performs with respect to the
variable loss circuits 25B and 25A in accordance with the
above-mentioned states 1 to 4, respectively.
Next, a description will be given of the operation of an embodiment
of the above construction. When a user of this transmitter-receiver
utters a vocal sound with the ear-piece type acoustic transducing
part 10 of FIG. 1 put on his or her ear, vibration of the skull as
well as aerial vibration are created by the vibration of the vocal
chords. The vibration of the skull is picked up as a bone-conducted
sound signal by the bone-conducted sound pickup microphone 14, from
11 which the signal is provided via the terminal T.sub.B to the
amplifier 21B. The aerial vibration of the speech is picked up by
the directional microphone 15, from which the signal is provided as
an air-conducted sound signal to the amplifier 21A via the terminal
T.sub.A.
In general, as compared with the air-conducted sound, the
bone-conducted sound has many low-frequency components, makes less
contribution to articulation and contains, in smaller quantity,
high-frequency components which are important for the expression of
consonants. On the other hand, abnormal sounds such as wind noise
caused by the wind blowing against the case 11 and frictional sound
between the cords (lead wires) 18 and the human body or clothing
are present in lower and higher frequency bands than the cutoff
frequencies of the filters 22A and 22B. Such wind noise and
frictional sounds constitute contributing factors to the lack of
articulation of the speech sending sound by the bone conduction and
the formation of abnormal sounds. On the other hand, "speech"
passes through the sound pickup tube 13 and is picked up as an
air-conducted sound signal by the directional microphone 15, from
which it is applied to the amplifier 21A via the terminal T.sub.A.
The air-conducted sound produced by a talker's speech is a human
voice itself, and hence contains frequency components spanning low
and high frequency bands.
In this embodiment, as described in the afore-mentioned Japanese
Utility Model Registration Application Laid-Open Gazette, the
high-frequency components of the bone-conducted sound from the
amplifier 21B are removed by the low-pass filter 22B to extract the
low-frequency components alone and the bone-conducted sound signal
thus having cut out therefrom the high-frequency components is
mixed with an air-conducted sound signal having cut out therefrom
the low-frequency components by the high-pass filter 22A. By this,
a speech sending signal is generated which has compensated for the
degradation of the articulation which would be caused by the lack
of the high-frequency components when the speech sending signal is
composed only of the bone-conducted sound signal. Besides,
according to the present invention, the processing for the
generation of such a speech sending signal is automatically
controlled to be optimal in accordance with each of the states
shown in FIG. 4, by which it is possible to generate a speech
sending signal of the best tone quality on the basis of
time-varying ambient noise and the speech transmitting-receiving
state.
The noise levels at the directional microphone 15 and the
omnidirectional microphone 16 can be regarded as about the same
level as referred to previously; but, because of a difference in
their directional sensitivity characteristic, the directional
microphone 15 picked up a smaller amount of noise energy than does
the omnidirectional microphone 16, and hence provides a higher SN
ratio. Since the gains G.sub.A and G.sub.U of the amplifiers 21A
and 21U are predetermined so that their output noise levels become
nearly equal to each other as mentioned previously, the gain
G.sub.A of the amplifier 21A is kept sufficiently larger than the
gain G.sub.U of the amplifier 21U. Hence, the user's speech signal
is amplified by the amplifier 21A with the large gain G.sub.A and
takes a level higher than the noise signal level.
The comparison/control circuit 24 compares, at regular time
intervals (1 sec, for instance), the outputs from the low-pass
filter 22B (for the bone-conducted sound) and the high-pass filter
22A (for the air-conducted sound) with the reference levels
V.sub.RB and V.sub.RA, respectively, to perform such control
operations as shown in FIG. 4. At first, the characteristic of the
transmitter-receiver of the present invention immediately after its
assembling is adjusted (or initialized) by setting the losses
L.sub.B and L.sub.A of the variable loss circuits 25B and 25A to
initial values L.sub.B0 and L.sub.A0 that the level of the
air-conducted sound signal to be input into the mixer 26 is higher
than the level of the bone-conducted sound signal by 3 to 10 dB
when no noise is present (State 4 in FIG. 4). The reason for this
is that it is preferable in terms of articulation that the
air-conducted sound be larger than the air-conducted one under
circumstances where no noise is present.
Next, a description will be given of the actual state of use in
which the levels of the bone- and air-conducted sound signals vary
every moment.
(a) When the output (the bone-conducted sound signal) from the
low-pass filter 22B is not present (State 3 or 4 in FIG. 4):
The comparison/control circuit 23 compares the output level V.sub.A
of the high-pass filter 22A with the reference level V.sub.RA. When
the output from the high-pass filter 22A is smaller than the
reference level V.sub.RA (State 4), the comparison/control circuit
23 decides that noise is not present or small and that no talks are
being carried out and sets the losses of the variable loss circuits
25B and 25A to the afore-mentioned initial values L.sub.B0 and
L.sub.A0, respectively. When this state changes to the talking
state (State 1), a mixture of the bone-conducted sound signal
composed of low-frequency components and the air-conducted sound
signal composed of high-frequency components is provided as the
speech sending signal S.sub.T at the output of the mixer circuit
26.
Next, when the output level V.sub.B of the low-pass filter 22B is
smaller than the reference level V.sub.RB and the output level
V.sub.A of the high-pass filter 22A is larger than the reference
level V.sub.RA (State 3), the comparison/control circuit 23 decides
that no talks are being carried out and that ambient noise is
large. In this instance, the comparison/control circuit 23 applies
a control signal C.sub.A to the variable loss circuit 25A to set
its loss L.sub.A to a value larger than the initial value L.sub.A0
in proportion to the difference between the output level V.sub.A of
the high-pass filter 22A and the reference level value V.sub.RA as
expressed by such an equation as follows:
where K is a predetermined constant. Alternatively, it is possible
to increase the loss L.sub.A by a constant K on a stepwise basis
each time the level difference (V.sub.A -V.sub.RA) increases by a
constant V.sub.M, as expressed by the following equation:
where [X] represents the smallest integer greater than x.
When the output from the low-pass filter 22B becomes larger than
the reference level V.sub.RB, that is, when this State 3 changes to
the talking state (State 1), the losses of the variable loss
circuits 25A and 25B are not changed but are kept at set values in
the immediately preceding State 3. By this, the bone-conducted
sound signal composed of low-frequency components and the
air-conducted sound signal of the same level as or lower than the
level of the bone-conducted sound signal and composed of
high-frequency components are mixed by the mixer circuit 26 into
the speech sending signal S.sub.T. In this case, it is also
possible to hold the loss of the variable loss circuit 25A
unchanged and control the loss of the variable loss circuit 25B so
that the mixed output level of the mixer circuit 26 takes a
predetermined value.
(b) When the output (the bone-conducted sound signal) level V.sub.B
of the low-pass filter 22B is larger than the reference level
V.sub.RB (State 1 or 2 in FIG. 4):
The comparison/control circuit 24 checks the output level V.sub.A
of the high-pass filter 22A and, if it is smaller than the
reference level V.sub.RA (State 2), determines that no talks are
being carried out and that the bone-conducted sound pickup
microphone 14 is picking up abnormal sounds. In such an instance,
the comparison/control circuit 24 applies a control signal C.sub.B
to the variable loss circuit 25B to set its loss L.sub.B to a value
greater than the initial value L.sub.B0 in proportion to the
difference between the output level V.sub.B of the low-pass filter
22B and the reference level V.sub.RA, as expressed by the following
equation:
Alternatively, as is the case with the above, the loss L.sub.B may
be controlled as expressed by the following equation:
When the output level V.sub.A of the high-pass filter 22A becomes
larger than the reference level V.sub.RA, that is, when this State
2 changes to the talking state (State 1), the losses of the
variable loss circuits 25A and 25B are held unchanged, and hence
are kept at the set values in the immediately preceding State 2. An
air-conducted sound signal composed of high-frequency components
and a bone-conducted sound signal of a level set in accordance with
the output level V.sub.B of the low-pass filter 22B and composed of
low-frequency components are mixed together by the mixer circuit
26. In this instance, it is also possible to hold the loss of the
variable loss circuit 25B unchanged and control the loss of the
variable loss circuit 25A so that the output level of the mixer
circuit 26 may assume the aforementioned predetermined fixed
value.
Next, when the output level V.sub.A of the high-pass filter 22a is
larger than the reference level V.sub.RA (State 1), the
comparison/control circuit 24 decides that the state is the talking
state, and causes the variable loss circuits 25B and 25A to hold
losses set in the state immediately preceding State 1. As a result,
bone- and air-conducted sound signals of levels controlled in
accordance with the losses held unchanged are mixed by the mixer
circuit 26, which provides the speech sending signal S.sub.T.
Incidentally, the variable loss circuits 29T and 29R and the
comparison/control circuit 28 are provided to suppress the
generation of an echo and howling which result from the coupling of
the speech sending system and the speech receiving system. The
ear-piece type acoustic transducing part 10 has the following two
primary contributing factors to the coupling which leads to the
generation of howling. First, when the transmitter-receiver
assembly is applied to a telephone set, a two-wire/four-wire
junction at a telephone station allows the speech sending signal to
sneak as an electrical echo into the speech receiving system from
the two-wire/four-wire junction, providing the coupling (sidetone)
between the two system. Second, a speech receiving signal is picked
up by the bone-conducted sound pickup microphone 14 or directional
microphone 15 as a mechanical vibration from the receiver 17 via
the case 11--this also provides the coupling between the two
systems. Such phenomena also occur in a loudspeaking telephone
system which allows its user to communicate through a microphone
and a loudspeaker without the need of holding a handset. In this
instance, however, the cause of the sneaking of the received sound
into the speech sending system is not the mechanical vibration but
the acoustic coupling between the microphone and the speaker
through the air.
This problem could be solved by known techniques such as a method
for the suppression of howling in the loudspeaking telephone
system. The configuration by the comparison/control circuit 28 and
the variable loss circuits 29T and 29R is an example of such a
prior art. The comparison/control circuit 28 monitors the output
level V.sub.T of the mixer circuit 26 and the signal level V.sub.R
at a received speech input terminal 20R and, when the speech
receiving signal level V.sub.R is larger than a predetermined level
V.sub.RR and the output level V.sub.T of the mixer circuit 26 is
smaller than a predetermined level V.sub.RT, the circuit 28 decides
that the transmitter-receiver is in the speech receiving state, and
sets a predetermined loss L.sub.T in the variable loss circuit 29T,
reducing the coupling of the speech receiving signal to the speech
sending system. When the output level V.sub.T of the mixer circuit
26 is larger than the predetermined level V.sub.RT and the input
level V.sub.R at the speech receiving signal input terminal 20R is
lower than the predetermined level V.sub.RR, the comparison/control
circuit 28 decides that the transmitter-receiver is in the talking
state, and sets a predetermined loss L.sub.R in the variable loss
circuit 29R, suppressing the sidetone from the speech receiving
system. When the output level V.sub.T of the mixer circuit 26 and
the input level V.sub.R at the speech receiving signal input
terminal 20R are higher than the predetermined levels V.sub.RT and
V.sub.RR, respectively, the comparison/control circuit 28 decides
that the transmitter-receiver is in a double-talk state, and sets
in the variable loss circuits 29T and 29R losses one-half those of
the above-mentioned predetermined values L.sub.T and L.sub.R,
respectively. In this way, speech with great clarity can be sent to
the other party in accordance with the severity of ambient noise
and the presence or absence of abnormal noise.
According to the first embodiment described above, a mixture of the
bone-conducted sound signal composed principally of low-frequency
components and the air-conducted sound signal composed principally
of high-frequency components is used as the speech signal that is
sent to the other party. Moreover, the ratio of mixture of the two
signals is automatically varied with the magnitude of ambient noise
and the abnormal sound picked up by the bone-conducted sound pickup
microphone. This permits the implementation of a
transmitter-receiver which can be used in a high-noise environment,
obviates such defects of the prior art as low clarity or
articulation and discomfort by abnormal sound, and allows
hands-free communications.
In the embodiment depicted in FIGS. 1 and 2, the comparison/control
circuit 24 and the variable loss circuits 25A and 25B may be
dispensed with, and even in such a case, the noise level can be
appreciably suppressed by the operations of the directional
microphone 15, the omnidirectional microphone 14 and the amplifiers
21A and 21B and the noise suppressing circuit 23 which form the
noise suppressing part 20N; hence, it is possible to obtain a
transmitter-receiver of higher speech quality than in the past.
Alternatively, the omnidirectional microphone 16, the amplifier 21U
and the noise suppressing circuit 23 may be omitted, and in this
case, too, the processing for the generation of the optimum speech
sending signal can automatically be performed by the operations of
the comparison/control circuit 24, the variable loss circuits 25A
and 25B and the mixer circuits 26 in accordance with the states of
signals involved.
Next, a detailed description will be given, with reference to FIGS.
5 through 9, of a second embodiment of the transmitter-receiver
according to the present invention.
FIG. 5 illustrates in block form the transmitter-receiver according
to the second embodiment of the invention. The bone-conducted sound
pickup microphone 14, the directional microphone 15 and the
receiver 17 are provided in such an ear-piece type acoustic
transducing part 10 as depicted in FIG. 1. In this embodiment, the
air-conducted sound signal from the directional microphone (the
air-conducted sound pickup microphone 15) and the bone-conducted
sound signal from the bone-conducted sound pickup microphone 14 are
fed to an air-conducted sound dividing circuit 31A and a
bone-conducted sound dividing circuit 31B via the amplifiers 21A
and 21B of the transmitting-receiving circuit 20, respectively. As
is the case with FIG. 2, the gains of the amplifiers 21A and 21B
are preset so that input air-and bone-conducted sound signals of a
vocal sound uttered in a no-noise environment have about the same
level. The air-conducted sound dividing circuit 31A divides the
air-conducted sound signal from the directional microphone 15 into
first through n-th frequency bands and applies the divided signals
to a comparison/control circuit 32 and signal select circuits
33.sub.1 through 33.sub.n. The bone-conducted sound dividing
circuit 31B divides the bone-conducted sound signal from the
bone-conducted sound pickup microphone 14 into first through n-th
frequency bands and applies the divided signals to the
comparison/control circuit 32 and the signal select circuits
33.sub.1 through 33.sub.n. In the present invention, the air- and
bone-conducted sound signals need not always be divided (i.e. n=1),
but when divided into frequency bands, they are divided, for
example, every one or one-third octave, or into high and low bands,
or high, intermediate and low bands.
A received signal dividing circuit 31R divides the received signal
S.sub.R from an external line circuit via the input terminal 20R
into first through n-th frequency bands and applies the divided
signal to the comparison/control circuit 32. In this embodiment,
the comparison/control circuit 32 is such one that converts each
input signal into a digital signal by an A/D converter (not shown),
and performs such comparison and control operations by a CPU (not
shown) as described below. That is, the comparison/control circuit
32 calculates an estimated value of the ambient noise level for
each frequency band on the basis of the air-conducted sound signals
of the respective bands from the air-conducted sound dividing
circuit 31A, the bone-conducted sound signals of the respective
bands from the bone-conducted sound dividing circuit 31B and the
received signals of the respective bands from the received signal
dividing circuit 31R. The comparison/control circuit 32 compares
the estimated values of the ambient noise levels with a
predetermined threshold value (i.e. a reference value for
selection) N.sub.th and generates control signals C.sub.1 to
C.sub.n for the respective bands on the basis of the results of
comparison. The control signals C.sub.1 to C.sub.n thus produced
are applied to the signal select circuits 33.sub.1 to 33.sub.n,
respectively. The signal select circuits 33.sub.1 to 33.sub.n
respond to the control signals C.sub.1 to C.sub.n to select the
air-conducted sound signals input from the air-conducted sound
dividing circuit 31A or the bone-conducted sound signals from the
bone-conducted sound signal dividing circuit 31B, which are
provided to a signal combining circuit 34. The signal combining
circuit 34 combines the input speech signals of the respective
frequency bands, taking into account the balance between the
respective frequency bands, and provides the combined signal to the
speech transmitting output terminal 20T. The output terminal 20T is
a terminal which is connected to an external line circuit.
FIG. 6 is a graph showing, by the solid lines 3A and 3B, a standard
or normal relationship between the tone quality (evaluated in terms
of the SN ratio or subjective evaluation) of the air-conducted
sound signal picked up by the directional microphone 15 and the
ambient noise level and a standard or normal relationship between
the tone quality of the bone-conducted sound signal picked up by
the bone-conducted sound pickup microphone and the ambient noise
level. The ordinate represents the tone quality of the sound
signals (the SN ratio in the circuit, for instance) and the
abscissa the noise level. As indicated by the solid line 3A, the
tone quality of the air-conducted sound signal picked up by the
directional microphone 15 is greatly affected by the ambient noise
level; the tone quality is seriously degraded when the ambient
noise level is high. On the other hand, as indicated by the solid
line 3B, the tone quality of the bone-conducted sound signal picked
up by the bone-conducted sound pickup microphone 14 is relatively
free from the influence of the ambient noise level; degradation of
the tone quality by the high noise level is relatively small.
Hence, the speech sending signal S.sub.T of good tone quality can
be generated by setting the noise level at the intersection of the
two solid lines 3A and 3B as the threshold value N.sub.th and by
selecting either one of the air-conducted sound signal picked up by
the directional microphone 15 and the bone-conducted sound signal
picked up by the bone-conducted sound pickup microphone, depending
upon whether the ambient noise level is higher or lower than the
threshold value N.sub.th. It was experimentally found that the
threshold value N.sub.th is substantially in the range of 60 to 80
dBA. The characteristics indicated by the solid lines 3A and 3B in
FIG. 6 are standard; the characteristics vary within the ranges
defined by the broken lines 3A' and 3B' in dependence upon the
characteristics of the microphones 14 and 15, the preset gains of
the amplifiers 21A and 21B and the frequency characteristics of the
input speech signals, but they remain in parallel to the solid
lines 3A and 3B, respectively. The solid lines 3A and 3B are
substantially straight.
The relationship between the tone quality of the air-conducted
sound signal by the directional microphone 15 and the ambient noise
level and the relationship between the tone quality of the
bone-conducted sound signal by the bone-conducted sound pickup
microphone 14 and the ambient noise level differ with the
respective frequency bands. For this reason, according to this
embodiment, the sound signals are each divided into respective
frequency bands and either one of the air- and bone-conducted sound
signals is selected depending upon whether the measured ambient
noise level is higher or lower than a threshold value set for each
frequency band--this provides improved tone quality of the speech
sending signal.
To switch between the air- and bone-conducted sound signals in
accordance with the ambient noise level, it is necessary to
calculate an estimated value of the ambient noise level. FIG. 7 is
a graph showing, by the solid line 4BA, a standard relationship of
the ambient noise level (on the abscissa) to the level ratio (on
the ordinate) between an ambient noise signal picked up by the
directional microphone 15 and an ambient noise signal picked-up by
the bone-conducted sound pickup microphone 14 in the listening or
speech receiving or silent states. FIG. 8 is a graph showing, by
the solid line 5BA, a standard relationship of the ambient noise
level to the level ratio between a signal (the air-conducted sound
signal plus the ambient noise signal) picked up by the directional
microphone 15 and a signal (the bone-conducted sound signal plus
the ambient noise signal) picked-up by the bone-conducted sound
pickup microphone 15 in the talking or double-talking state. As
shown in FIGS. 7 and 8, the characteristic in the listening or
silent state and the characteristic in the talking or
double-talking state differ from each other. Hence, the level
V.sub.A of the air-conducted sound signal from the directional
microphone 15, the level V.sub.B of the bone-conducted sound signal
from the bone-conducted sound pickup microphone 15 and the level
V.sub.R of the received signal from the amplifier 27 are compared
with the reference level values V.sub.RA, V.sub.RB and V.sub.RR,
respectively, to determine if the transmitter-receiver is in the
listening (or silent) state or in the talking (or double-talking)
state. Next, the level ratio V.sub.B /V.sub.A between the
bone-conducted sound signal and the air-conducted sound signals
picked up by the microphones 14 and 15 in the listening or silent
state is calculated, and the noise level at that time is estimated
from the level ratio through utilization of the straight line 4BA
in FIG. 7. Depending upon whether the estimated noise level is
higher or lower than the threshold value N.sub.th in FIG. 6, the
signal select circuits 33.sub.1 to 33.sub.n each select the
bone-conducted sound signal or air-conducted sound signal.
Similarly, the level ratio V.sub.B /V.sub.A between the
bone-conducted sound signal and the air-conducted sound signal in
the talking or double-talking state is calculated, then the noise
level at that time is estimated from the straight line 5BA in FIG.
8, and the bone-conducted sound signal or air-conducted sound
signal is similarly selected depending upon whether the estimated
noise level is above or below the threshold value N.sub.th.
Next, the operation of the transmitter-receiver will be described.
Incidentally, let is be assumed that there are prestored in a
memory 32M of the comparison/control circuit 32 the reference level
values V.sub.RA, V.sub.RB and V.sub.RR, the threshold value
N.sub.th and the level ratio vs. noise level relationships shown in
FIGS. 7 and 8. Since the speech signals and the received signals
divided into the first through n-th frequency bands are subjected
to exactly the same processing until they are input into the signal
combining circuit 34, the processing in only one frequency band
will be described using reference numerals with no suffixes
indicating the band.
The comparison/control circuit 32 compares, at regular time
intervals (of one second, for example), the levels V.sub.A, V.sub.B
and V.sub.R of the air-conducted sound signal, the bone-conducted
sound signal and the received signal input from the air-conducted
sound dividing circuit 31A, the bone-conducted sound dividing
circuit 31B and the received signal dividing circuit 31R with the
predetermined reference level values V.sub.RA, V.sub.RB and
V.sub.RR, respectively. When the level V.sub.R of the received
signal S.sub.R is higher than the predetermined value V.sub.RR and
the level V.sub.A of the air-conducted sound signal picked up by
the directional microphone 15 and the level V.sub.B of the
bone-conducted sound signal picked up by the bone-conducted sound
pickup microphone 14 are smaller than the predetermined values
V.sub.RA and V.sub.RB, respectively, the comparison/control circuit
32 determines that this state is the listening state shown in the
table of FIG. 9. When the level V.sub.R of the received signal
level V.sub.R is smaller than the predetermined value V.sub.RR and
the levels V.sub.A and V.sub.B of the air-conducted sound signal
and the bone-conducted sound signal are both smaller than the
predetermined values V.sub.RA and V.sub.RB, the circuit 32
determines that this state is the silent state. In these two states
the comparison/control circuit 32 calculates the level ratio
V.sub.B /V.sub.A between the air-conducted sound signal from the
air-conducted sound dividing circuit 31A and the bone-conducted
sound signal from the bone-conducted sound dividing circuit 31B.
Based on the value of this level ratio, the comparison/control
circuit 32 refers to the relationship of FIG. 7 stored in the
memory 32M to obtain an estimated value of the corresponding
ambient noise level. When the estimated value of the ambient noise
level is smaller than the threshold value N.sub.th shown in FIG. 6,
the comparison/control circuit 32 supplies the signal select
circuit 33 with a control signal C instructing it to select and
output the air-conducted sound signal input from the air-conducted
sound dividing circuit 31A. When the estimated value of the ambient
noise level is greater than the threshold value N.sub.th, the
comparison/control circuit 32 applied the control signal C to the
signal select circuit 33 to instruct it to select and output the
bone-conducted sound signal input from the bone-conducted sound
dividing circuit 31B.
On the other hand, when the received signal level V.sub.R is
smaller than the reference level value V.sub.RR and the levels
V.sub.A and V.sub.B of the air-conducted sound signal by the
directional microphone 15 and the bone-conducted sound signal by
the bone-conducted sound pickup microphone 14 are larger then the
predetermined reference level values V.sub.RA and V.sub.RB, the
comparison/control circuit 32 determines that this state is the
talking state shown in the table of FIG. 9. When the received
signal level V.sub.R is larger than the reference level value
V.sub.RR and the levels V.sub.A and V.sub.B of the air-conducted
sound signal and the bone-conducted sound signal are larger than
the predetermined reference level values V.sub.RA and V.sub.RB, the
comparison/control circuit 32 determines that this state is the
double-talking state. In these two states the comparison/control
circuit 32 calculates the level ratio V.sub.B /V.sub.A between the
bone-conducted sound signal and the air-conducted sound signal and
estimates the ambient noise level N through utilization of the
relationship of FIG. 8 stored in the memory 32M.
When the thus estimated value of the ambient noise level N is
smaller than the threshold value N.sub.th shown in FIG. 6, the
comparison/control circuit 32 applies the control signal C to the
signal select circuit 33 to cause it to select and output the
air-conducted sound signal input from the air-conducted sound
dividing circuit 31A. When the estimated value N of the ambient
noise level is greater than the threshold value N.sub.th, the
circuit 32 applies the control signal C to the signal select
circuit 33 to cause it to select and output the bone-conducted
sound signal input from the bone-conducted sound dividing circuit
31B.
The comparison/control circuit 32 has, in the memory 32M for each
of the first through n-th frequency bands, the predetermined
threshold value N.sub.th shown in FIG. 6 and the level ratio vs.
noise level relationships representing the straight characteristic
lines 4BA and 5BA shown in FIGS. 7 and 8. The comparison/control
circuit 32 performs the same processing as mentioned above and
applies the resulting control signals C.sub.1 to C.sub.n to the
signal select circuits 33.sub.1 to 33.sub.n. The signal combining
circuit 34 combines the speech signals from the signal select
circuits 33.sub.1 to 33.sub.n, taking into account the balance
between the respective frequency bands.
While in the above the embodiments have been described to estimate
and compare the noise level with the threshold value and control
the signal select circuits 33.sub.1 to 33.sub.n accordingly in any
state described in the table of FIG. 9, it is also possible to
employ a scheme that estimates the noise level only in the silent
or listening state and uses the thus estimated noise level to
effect control in the talking state and the double-taking state. In
such an instance, the characteristic data of FIG. 8 need not be
stored in the memory 32M. In contrast to this, the estimation of
the noise level may be made only in the talking or double-talking
state, in which case the estimated noise level is used for control
in the talking or double-talking state. In this instance, the
characteristic data of FIG. 7 is not needed.
Incidentally, the double-talking state duration and the silent
state duration are shorter than the talking or listening state
duration. Advantage may also be taken of this to effect control in
the double-talking state and in the silent state by use of the
ambient noise level estimated prior to these states.
When the level of the bone-conducted sound signal picked up by the
bone-conducted sound pickup microphone 14 is abnormally high, it
can be considered that the signal includes noise made by the
friction of cords or the like; hence, it is effective to select the
air-conducted sound signal picked up by the directional microphone
15.
In the case where the estimated noise level N is compared with the
threshold value N.sub.th for each frequency band and the
air-conducted sound signal picked up by the directional microphone
15 is switched to the bone-conducted sound signal by the
bone-conducted sound pickup microphone 14 on the basis of the
result of comparison as described previously with reference to the
FIG. 5 embodiment, the timbre of the speech being sent may
sometimes undergo an abrupt change, making the speech unnatural. To
solve this problem, an area N.sub.W of a fixed width as indicated
by N.sup.- and N.sup.+ is provided about the threshold value
N.sub.th of the ambient noise level shown in FIG. 6; when the
estimated noise level N is within the area N.sub.W, the
air-conducted sound signal from the directional microphone 15 and
the bone-conducted sound signal from the bone-conducted sound
pickup microphone 14 are mixed in a ratio corresponding to the
noise level, and when the estimated noise level N is larger than
the area N.sub.W, the bone-conducted sound signal is selected, and
when the estimated noise level is smaller than the area N.sub.W,
the air-conducted sound signal is selected. By this, it is possible
to reduce the abrupt change in the timbre prior to or subsequent to
the switching operation.
The modification of the FIG. 5 embodiment for such signal
processing can be effected by using, for example, a signal mixer
circuit 33 depicted in FIG. 10A in place of each of the signal
select circuits 33.sub.1 to 33.sub.n. In this example, the
corresponding air-conducted sound signal and bone-conducted sound
signal of each frequency band are applied to variable loss circuits
33A and 33B, respectively, wherein they are given losses L.sub.A
and L.sub.B set by control signals C.sub.A and C.sub.B from the
comparison/control circuit 32. The two signals are mixed in a mixer
33C and the mixed signal is applied to the signal combining circuit
34 in FIG. 5.
The losses L.sub.A and L.sub.B for the air-conducted sound signal
and the bone-conducted sound signal in the area N.sub.W need only
be determined as shown in FIG. 10B, for instance. For brevity's
sake, setting N.sub.th =(N.sup.+ +N.sup.-)/2, the area width to
D=N.sup.+ -N.sup.-, the minimum values L.sub.A0 and L.sub.B0 of the
losses L.sub.A and L.sub.B to 0 dB, respectively, and their maximum
values L.sub.AMAX and L.sub.BMAX to the same L.sub.MAX dB, the loss
L.sub.A in the area N.sub.W can be expressed, for example, by the
following equation: ##EQU1## Similarly, the loss L.sub.B can be
expressed by the following equation: ##EQU2## The value of the
maximum loss L.sub.MAX is selected in the range of between 20 and
40 dB, and the width D of the area N.sub.W is set to about 20 dB,
for instance. When the estimated noise level N is larger than the
area N.sub.W, the bone-conducted sound signal is not given any loss
(L.sub.B =0) and is applied intact to the mixer 33C. On the other
hand, the air-conducted sound signal is not given the loss
L.sub.MAX but instead the variable loss circuit 33A is opened to
cut off the signal. Similarly, when the estimated noise level N is
smaller than the area N.sub.W, the air-conducted sound signal is
not given any loss (L.sub.A =0) and is fed intact to the mixer 33C,
whereas the bone-conducted sound signal is cut off by opening the
variable loss circuit 33B. The comparison/control circuit 32
determines the losses L.sub.A and L.sub.B for each band as
described and sets the losses in the variable loss circuits 33A and
33B by the control signals C.sub.A and C.sub.B.
With such signal processing as described above, it is possible to
provide smooth timbre variations of the speech being sent when the
air-conducted sound signal is switched to the bone-conducted sound
signal or vice versa. Moreover, if the levels of the air-conducted
sound signal and the bone-conducted sound signal input into the
variable loss circuits 33A and 33B are nearly equal to each other,
the output level of the mixer 33C is held substantially constant
before and after the switching between the air- and bone-conducted
sound signals and the output level in the area N.sub.W is also held
substantially constant, ensuring smooth signal switching.
Incidentally, the signal select processing by the signal select
circuits 33.sub.1 to 33.sub.n in FIG. 5 corresponds to the case
where the width D of the area N.sub.W is set to zero in the
processing in the modified embodiment depicted in FIGS. 10A and
10B. Hence, it can be said, in a broad sense, that the signal
select circuits 33.sub.1 to 33.sub.n also contribute to the mixing
of signals on the basis of the estimated noise level.
In the above, when the estimation of the ambient noise level may be
rough, it can be estimated by using average values of the
characteristics shown in FIGS. 7 and 8. In this instance, the
received signal dividing circuit 31R can be dispensed with. When
the estimation of the ambient noise level may be rough, it can also
be estimated by using only the speech signal from the directional
microphone 14.
FIG. 11 illustrates in block diagram a modified form of the FIG. 5
embodiment, in which as is the case with the first embodiment of
FIGS. 1 and 2, the omnidirectional microphone 16, the amplifier 21U
and the noise suppressing circuit 23 are provided in association
with the direction microphone 15 and the output from the noise
suppressing circuit 23 is fed as an air-conducted sound signal to
the air-conducted sound dividing circuit 31A. This embodiment is
identical in construction with the FIG. 5 embodiment except for the
above. In this embodiment, when the transmitter-receiver is in the
silent or listening state, a switch 35 is opened and only the
air-conducted sound signal provided via the amplifier 21U from the
omnidirectional microphone 16 is applied to the noise suppressing
circuit 23, from which it is fed intact to the air-conducted sound
dividing circuit 31A, and the air-conducted sound signals divided
into respective frequency bands are applied to the
comparison/control circuit 32. As in the FIG. 5 embodiment, the
comparison/control circuit 32 estimates the ambient noise levels
through utilization of the relationships shown in FIG. 7 and, based
on the estimated levels, generate the control signals C.sub.1 to
C.sub.n for signal selection (or mixing use in the case of using
the FIG. 10A circuit configuration), which are applied to the
signal select circuits 33.sub.1 to 33.sub.n (or the signal mixing
circuit 36). After this, the switch 35 is turned ON to pass
therethrough the air-conducted sound signal from the directional
microphone 15 to the noise suppressing circuit 23, in which its
noise components are suppressed, and then the air-conducted sound
signal is fed to the air-conducted sound dividing circuit 31A. This
is followed by the speech sending signal processing by the same
signal selection or mixing as described previously with respect to
FIG. 5.
Although in the embodiments of FIGS. 5 and 11 the
comparison/control circuit 32 has been described to convert the
signals input thereto to digital signals and generate the control
signals C.sub.1 to C.sub.n on the basis of the level ratio-noise
level relationships stored in the memory 32M, the
comparison/control circuit 32 may also be formed as an analog
circuit, for example, as depicted in FIG. 12. In FIG. 12 there is
shown in block form only a circuit portion corresponding to one of
the divided subbands. A pair of corresponding subband signals from
the air-conducted sound signal dividing circuit 31A and the
bone-conducted sound signal dividing circuit 31B are both applied
to a level ratio circuit 32A and a comparison/logic state circuit
32E. The level ratio circuit 32A calculates the level ratio L.sub.B
/L.sub.A between the bone- and air-conducted sound signals in an
analog fashion and supplies level converter circuits 32B and 32C
with a signal of a level corresponding to the calculated level
ratio.
The level converter circuit 32B performs a level conversion based
on the relationship shown in FIG. 7. That is, when supplied with
the level ratio V.sub.B /V.sub.A, the level converter circuit 32B
outputs an estimated noise level N corresponding thereto and
provides it to a select circuit 32D. Similarly, the level converter
circuit 32C performs a level conversion based on the relationship
shown in FIG. 8. That is, when supplied with the level ratio
V.sub.B /V.sub.A, the level converter circuit 32C outputs an
estimated noise level corresponding thereto and provides it to the
select circuit 32D. On the other hand, the comparison/state logic
circuit 32E compares the levels of the corresponding air- and
bone-conducted sound signals of the same subband and the level of
the received speech signal with the reference levels V.sub.RA,
V.sub.RB and V.sub.RR, respectively, to make a check to see if
these signals are present. Based on the results of these checks,
the comparison/state logic circuit 32E applies a select control
signal to the select circuit 32D to cause it to select the output
from the level converter circuit 32B in the case of State 1 or 2
shown in the table of FIG. 9 and the output from the level
converter circuit 32C in the case of State 3 or 4.
The select circuit 32D supplies a comparator circuit 32F with the
estimated noise level N selected in response to the select control
signal. The comparator circuit 32F compares the estimated noise
level N with the threshold level N.sub.th and provides the result
of the comparison, as a control signal C for the subband concerned,
to the corresponding one of the signal select circuits 31.sub.1 to
31.sub.n in FIG. 5 or 11. In this instance, it is also possible to
make a check to determine if the estimated noise level N is within
the area N.sub.W or higher or lower than it as described previously
with respect to FIG. 10B, instead of comparing the estimated noise
level N with the threshold value N.sub.th ; if the estimated noise
level N is within the area N.sub.W, the control signals C.sub.A and
C.sub.B corresponding to the difference between the estimated noise
level N and the threshold level N.sub.th, as is the case with Eqs.
(5) and (6), are applied to the signal mixing circuit of the FIG.
10A configuration to cause it to mix the air-conducted sound signal
and the bone-conducted sound signal; when the estimated noise level
N is higher than the area N.sub.W, the bone-conducted sound signal
is selected and when the estimated noise level N is lower than the
area N.sub.W, the air-conducted sound signal is selected.
As described above, according to the transmitter-receiver of the
embodiment shown in each of FIGS. 5 and 11, the air-conducted sound
signal picked up by the directional microphone and the
bone-conducted sound signal picked-up by the bone-conducted sound
pickup microphone are used to estimate the ambient noise level and,
on the basis of the magnitude of the estimated noise level, one of
the air-conducted sound signal and the bone-conducted sound signal
is selected or both of the signals are mixed together, whereby a
speech sending signal of the best tone quality can be generated.
Thus, the communication device of the present invention is able to
transmit speech sending signals of excellent tone quality,
precisely reflecting the severity and amount of ambient noise
regardless of whether the device is in the talking or listening
state.
While in the first and second embodiments the
transmitting-receiving circuit 20 is described to be provided
outside the case 11 of the ear-piece type acoustic transducing part
10 and connected thereto via the cord 18, it is evident that the
transmitting-receiving circuit 20 may be provided in the case 11 of
the acoustic transducing part 10.
It will be apparent that many modifications and variations may be
effected without departing from the scope of the novel concepts of
the present invention.
* * * * *